The invention relates to a rear support balance for measuring the six components of the flight mechanical axis system for aerodynamic force determination on wind tunnel models with a balance body inserted between the model and the mounting and having an action beam and a reaction beam between which are arranged measurement pickups the respective output electrical charge of which is a measure of the respective component.
Models of aircraft, projectiles and land vehicles are conventionally investigated in wind tunnels. A uniform air stream is produced by the wind tunnel and acts on the model. For this purpose, the model is arranged in a measuring section of the wind tunnel. The most frequent type of fastening for the model consists of a round rod known as a rear support which is introduced through the rear of the model and is clamped fast in the fuselage of the model so that the model is carried by it. At the other end, the rear support is received in a mounting by means of which the setting of the model relative to the flow can be varied.
The essential aim of model investigations in wind tunnels is the determination of the aerodynamic force resulting from the flow around the model, and of the moment of the aerodynamic force. Corresponding to the three main axis directions of an aircraft, the aerodynamic force and the aerodynamic force moment can be resolved, corresponding to the flight mechanics axis system, into three mutually perpendicular force components and three moment components about these axes. The determination of these aerodynamic force components is effected with a rear support balance.
The rear support balances used today, of the kind previously described, have a balance body consisting of an action beam and a reaction beam, which are connected by thin, deformable material elements. Strain gauges are mounted in the region of these deformable pieces of material. The aerodynamic forces cause a slight deformation of the deformable material parts and of the strain gauges on them. The strain gauges are arranged in a bridge circuit. Deformations proportional to the forces bring about a detuning of these bridges in which the strain gauges are located. Strain gauges are passive measurement pickups to which, as is known, a supply voltage has to be applied. The voltages arising on deformation, which can be taken off, are approximately linearly proportional to the aerodynamic forces or aerodynamic force moments.
With the known rear support balances, useful measurement results are obtained in stationary flows at constant or only slightly varying measurement temperatures. These balances are particularly suitable for static long-term investigations. Rear support balances operating with strain gauges, however, have available only a low stiffness, dependent on the tangential force element. This often leads to difficulties with the system of model and balance in the case of measurements in the high angle of attack region. The consequences are false measurement results or damage to the balance. For dynamic measurements and for measurements in non-stationary flows, the known balances are of only limited usefulness. Furthermore, local temperature gradients can bring about considerable measurement errors.